During recent years, olefin metathesis has seen an extraordinary development and has turned out to be a very versatile and efficient tool in organic synthesis.
The success of the olefin metathesis reaction is mainly attributed to the versatility and the development of well-defined Ruthenium catalysts stable to demanding reaction conditions. As these catalysts became commercially available and were exposed to a myriad of potentially interesting applications, the field was faced with renewed challenges, e.g. catalyst latency. The ideal latent olefin metathesis catalyst exhibits no catalytic activity in the presence of monomer or substrate at room temperature, but can be triggered quantitatively to a highly active form by thermal, chemical or photochemical activation to initiate the metathesis reaction. Additionally, catalyst stability towards decomposition or thermal degradation should be guaranteed by the rigorous choice of ligand environment.
Industrial application in DCPD polymerization requires the latent catalysts exhibiting decreased initiation rates, which can allow for longer handling of a monomer-catalyst mixture before the polymerization starts.
Van der Schaaf and co-workers developed the temperature activated, slow initiating olefin metathesis catalyst (PR3)(Cl)2Ru(CH(CH2)2—C,N-2-C5H4N) (Scheme 1) in which initiation temperatures were tuned by changing the substitution pattern of the pyridine ring (Van der Schaaf, P. A.; Kolly, R.; Kirner, H.-J.; Rime, F.; Mühlebach, A.; Hefner, A. J. Organomet. Chem. 2000, 606, 65-74). Unfortunately, activities of the reported complexes were undesirably low; restricted to 12000 equiv DCPD. Later, Ung reported on analogous tuneable catalytic systems obtained by partially isomerising trans-(SIMes)(Cl)2Ru(CH(CH2)2—C,N-2-C5H4N) (2) into the cis analogue (, T.; Hejl, A.; Grubbs, R. H.; Schrodi, Y. Organometallics 2004, 23, 5399-5401). However, none of these catalysts allowed for storage in DCPD monomer for long time as the ROMP of DCPD is completed in 25 minutes after catalyst introduction.

In another approach towards rationally designed thermally stable olefin metathesis catalyst for DCPD polymerization, efforts were directed towards the development of an O,N-bidentate Schiff base ligated Ru-carbene catalysts elaborated by Verpoort et al. (Scheme 2, 4, 5, L=SIMes). It was shown that such complexes are extremely inactive at room temperature towards the polymerization of low-strain, cyclic olefins, allow for storage in DCPD for months and can be thermally activated to yield increased activity for the bulk-polymerization of DCPD, but activities comparable to the corresponding complexes without Schiff bases could not be reached (EP 1 468 004; Allaert, B.; Dieltiens, N.; Ledoux, N.; Vercaemst, C.; Van Der Voort, P.; Stevens, C. V.; Linden, A.; Verpoort, F. J. Mol. Cat. A: Chem. 2006, 260, 221-226).

Additionally, activation of the catalyst was facilitated by the addition of high amounts of Bronsted acids (e.g. HCl) leading to high catalytic activity for the ROMP of DCPD (EP 1 577 282; EP 1 757 613; B. De Clercq, F. Verpoort, Tetrahedron Lett., 2002, 43, 9101-9104; (b) B. Allaert, N. Dieltens, N. Ledoux, C. Vercaemst, Van Der Voort, C. V. Stevens, A. Linden, F. Verpoort, J. Mol. Catal. A: Chem., 2006, 260, 221-226; (c) N. Ledoux, B. Allaert, D. Schaubroeck, S. Monsaert, R. Drozdzak, P. Van Der Voort, F. Verpoort, J. Organomet. Chem., 2006, 691, 5482-5486). However, the requirement of the high amounts of HCl, due its high volatility and corrosion problems prevents them from being industrially applicable.
Recently a series of latent olefin metathesis catalysts bearing 1bidentate κ2-(O,O) ligands were synthesized (Scheme 2, 3). Complex 3, proved to be inactive for the solvent-free polymerization of DCPD. It was furthermore illustrated that complex 3 (Scheme 2, L=PCy3, SIMes) is readily activated upon irradiation of a catalyst/monomer mixture containing a photoacid generator and was found applicable in ROMP of DCPD (D. M. Lynn, E. L. Dias, R. F L Grubbs, B. Mohr, 1999, WO 99/22865). Nevertheless irradiation of a solution of DCPD, 3 (L=SIMes) in a minimal amount of CH2Cl2 resulted in complete gelation within 1 h but solidified and Cross-linked monomer was not obtained. This indicates low catalyst activity and the operation on a low amount of the active species. Moreover the synthetic protocol for catalyst 3 is saddled with a serious drawback, namely the use of a Tl(alkyl-acac). Thallium and its derivatives are extremely toxic; consequently, the use of this procedure is not industrially applicable. In addition, the use of Ag(Me6acac) resulted in complete ligand exchange, but the desired product 3 resisted all attempts at further purification, only ligand exchange using thallium as a more capable transmetalation element provided the desired complex 3 cleanly and in excellent yield (K. Keitz, R. H. Grubbs, J. Am. Chem. Soc., 2009, 131, 2038-2039).
Summarizing, the latent catalysts are of prominent importance for Ring-Opening Metathesis Polymerizations of low-strained cyclic olefins, as they allow for mixing of monomer and catalyst without concomitant gelation or microencapsulation of the precatalyst. Production of a latent catalyst stable in the monomer, highly active after an industrially acceptable activation process and obtained by using environmentally friendly procedure remains challenging.